computer system been taken over by 7-zip

Had been trying to open a game with 7-zip and used file menu tools, by mistake pressed add to notepad.
Everything has changed to 7-zip even when l press google chrome. Tried command promt,readit but goes straight to another 7-zip box again.In my file/protocal under .sys again programme is 7-zip. l am not a wiz and completely stuck.please help